1. Field of the Invention
The present invention relates to a transferring apparatus that is used in an image forming apparatus.
2. Related Background Art
A transferring apparatus is used in an image forming apparatus according to an electrophotographic process in order to transfer a toner image borne on an image bearing member, a so-called photosensitive drum, to a transferring material, a so-called sheet. There are several types of apparatuses known as transferring apparatuses.
Among the transferring apparatuses, a system is widely used for transferring a toner image borne on a photosensitive drum to a sheet by applying a transfer voltage to a cylindrical transferring member, a so-called transfer roller, and passing a sheet between the transfer roller and the photosensitive drum. The transfer roller and the photosensitive drum are in contact in this system in a state in which a sheet has not passed between the transfer roller and the photosensitive drum. The sheet therefore remains within the apparatus, and the transfer roller may be contaminated by the toner when the sheet is removed manually, or the like. Accordingly, this system has a function for cleaning the transfer roller by applying a voltage having a polarity opposite to that of the transfer voltage to the transfer roller at a predetermined timing, and rotating the photosensitive drum and the transfer roller.
A circuit that generates a positive transfer voltage and a circuit that generates a negative transfer voltage are provided in a transfer voltage generator circuit. Direct current high voltage output circuits that are structured by an inverter transformer and a rectifying circuit are generally used as the circuits that respectively generate the positive transfer voltage and the negative transfer voltage. A positive electric potential voltage is variably output as the transfer voltage with this type of transfer voltage generator circuit, the voltage varying according to the environment and transfer roller characteristics. On the other hand, an output voltage used when cleaning the transfer roller is a negative voltage in order to achieve a function for promoting the toner to move from the transfer roller to the photosensitive drum. High precision is not demanded for the negative voltage, and therefore variable voltage control is not necessary. The negative voltage is a fixed output voltage.
The transfer voltage generator circuit is explained while referring to FIGS. 6 to 8. FIG. 6 is a schematic diagram that shows a transfer voltage generator circuit for a case where the toner is negative toner, and FIG. 7 shows a pulse waveform that is output from a pulse output port DPLS10 of a microcomputer IC201 of FIG. 6. FIG. 8 is a graph that shows a relationship between an output voltage of a positive transfer voltage generator circuit 202 and a PWM (pulse width modulation) signal output from the microcomputer IC201 of FIG. 6.
A photosensitive drum 105 that is scanned and exposed by a laser light 109 is provided in an image forming apparatus as shown in FIG. 6, and the photosensitive drum 105 is grounded. A charging roller 107, a developing sleeve 108, and a transfer roller 106 are disposed in the periphery of the photosensitive drum 105. Predetermined voltages are applied to the charging roller 107 and to the developing sleeve 108 by a charging voltage generator circuit (not shown) and a developing voltage generator circuit (not shown), respectively. A transfer voltage that is output from the transfer voltage generator circuit 201 is applied to the transfer roller 106.
The photosensitive drum 105 is rotated in a direction of an arrow in FIG. 6 when forming an image, and a surface of the photosensitive drum 105 is charged uniformly to a predetermined electric potential by the charging roller 108. The surface of the photosensitive drum 105 is then scanned and exposed by the laser light 109. An electrostatic latent image is thus formed on the photosensitive drum 105. The electrostatic latent image is then made into a visible image as a toner image by toner supplied from the developing sleeve 108. The toner image borne on the photosensitive drum 105 is transferred by the transfer roller 106 onto a sheet 110 that is nipped and conveyed between the photosensitive drum 105 and the transfer roller 106.
The transfer voltage generator circuit 201 has the microcomputer IC201, the positive transfer voltage generator circuit 202 that generates the positive transfer voltage, a negative transfer voltage generator circuit 103 that generates the negative transfer voltage, and a transfer current detector circuit 104 that detects current flowing in the transfer roller 106. The microcomputer IC201 has two independent output ports DPLS10, one port PWM, and one A/D port CRINT. Pulses having the same waveform are output from the two pulse output ports DPLS10. Both of the waveforms are waveforms having an ON duty of 10%, for example, as shown in FIG. 7. The two pulses serve as drive signals for the positive transfer voltage generator circuit 201 and the negative transfer voltage generator circuit 103, and drive inverter transformers T101 and T102, respectively. Outputs from the inverter transformers T101 and T102 are changed into the positive transfer voltage and the negative transfer voltage through a latter stage quadruple rectifying circuit and a latter stage rectifying circuit, respectively. That is, the microcomputer IC201 turns on the pulse output port DPLS10 that is connected to the positive transfer voltage generator circuit 201 when outputting the positive transfer voltage. The microcomputer IC201 turns on the pulse output port DPLS10 that is connected to the negative transfer voltage generator circuit 103 when outputting the negative transfer voltage.
The PWM port is connected to the positive transfer voltage generator circuit 202, and the A/D port is connected to the transfer current detector circuit 104. A current value detected by the transfer current detector circuit 104 is input to the microcomputer IC201 through the A/D port, and the microcomputer IC201 determines the transfer voltage based on the current value. The PWM signal is changed and sent to the positive transfer voltage generator circuit 202 through the port PWM to obtain the determined transfer voltage. A driver voltage of the transformer T101 of the positive transfer voltage generator circuit 202 is changed according to the PWM signal, and the desired output voltage (transfer voltage) is obtained. For example, a relationship between the output voltage of the positive transfer voltage generator circuit 202 and the value set for the PWM signal is shown in FIG. 8 when the PWM signal is variable to 256 levels.
The positive transfer voltage generator circuit 202 specifically includes a switching portion that drives the transformer T101 based on the pulse signal from the pulse output port DPLS10 of the microcomputer IC201, a constant voltage control portion that controls the switching state of the transformer T101, and a quadruple rectifier portion that rectifies and smoothes the output voltage of the transformer T101. The switching portion is constituted of transistors Q101 and Q102, resistors R101 and R102, a capacitor C202, and a diode D101.
The constant voltage control portion is constituted of a comparative operational amplifier IC 202, a transistor Q201, resistors R201, R202, R203, R204, R205, and R103, and a capacitor C201. A voltage to be input to the comparative operational amplifier IC 202 is generated in the constant voltage control portion based on the PWM signal from the microcomputer IC201. An operation of the transistor Q201 is controlled based on the results of the comparison operation of the comparative operational amplifier IC 202.
The quadruple rectifier portion is constituted of capacitors C101, C102, C103, and C104, diodes D102, D103, D104, and D105, and a resistor R104. The output voltage of the rectifier portion is a positive voltage, and the output voltage is applied to the transfer roller 106, which is a load.
The negative transfer voltage generator circuit 103 specifically includes a switching portion that drives the transformer T102 based on the pulse signal from the pulse output port DPLS10 of the microcomputer IC201, and a rectifier portion that rectifies and smoothes the output voltage of the transformer T102. The switching portion is constituted of transistors Q103 and Q104, and resistors R105, R106, and R107. The resistor R107 is connected to a reference power source (24 V) here, and the output voltage of the transformer T102 is set by the reference power source. The rectifier portion is constituted of a capacitor C105, a diode D107, and a resistor R108. The output voltage of the rectifier portion is a negative voltage, and the output voltage is applied to the transfer roller 106, which is a load.
The transfer current detector circuit 104 detects the value of the current that flows in the transfer roller 106 when the positive output voltage of the positive transfer voltage generator circuit 202 is applied to the transfer roller 106. The detected current value is sent to the microcomputer IC201. The transfer current detector circuit 104 is specifically constituted of a comparative operational amplifier IC102, capacitors C106 and C107, and resistors R109, R110, R111, R112, R113, R114, R115, and R116. Output from the comparative operational amplifier IC102 is input to the microcomputer IC201 as a signal (CRNT) that shows the detected current value.
Further, it is also possible to use a circuit as disclosed in Japanese Patent Application Laid-Open No. H08-140351, which changes a driving frequency of an inverter transformer as the positive transfer voltage generator circuit. A transfer voltage generator circuit that adopts the circuit disclosed in Japanese Patent Application Laid-Open No. H08-140351 as the positive transfer voltage generator circuit is explained while referring to FIGS. 9 to 11. FIG. 9 is a diagram that shows a circuit configuration of a transfer voltage generator circuit that adopts the circuit disclosed in Japanese Patent Application Laid-Open No. H08-140351 as a positive transfer voltage generator circuit. FIG. 10 shows a pulse waveform output from a port DPLSVAR of a microcomputer IC301 of FIG. 9. FIG. 11 is a graph that shows a relationship between an output voltage of a positive transfer voltage generator circuit 102 and the pulse output from the port DPLSVAR of the microcomputer IC301 of FIG. 9. It should be noted that elements shown in FIG. 9 which are identical to the circuits, components, and members shown in FIG. 6 are denoted by the same reference symbols as those used in FIG. 6.
Specifically, the transfer voltage generator circuit 301 has the positive transfer voltage generator circuit 102, the negative transfer voltage generator circuit 103, the transfer current detector circuit 104, and the microcomputer IC301 as shown in FIG. 9. The positive transfer voltage generator circuit 102 includes a switching portion that drives the transformer T101 based on the pulse signal from the port DPLSVAR of the microcomputer IC301, and a quadruple rectifier portion that rectifies and smoothes the output voltage of the transformer T101. The switching portion is constituted of the transistors Q101 and Q102, the resistors R101, R102, and R103, and the diode D101. The resistor R103 is connected to a reference power source (24 V) here, and the output voltage of the transformer T102 is set by the reference power source.
The microcomputer IC301 has the port DPLSVAR for outputting a pulse with a variable frequency and fixed on-time, one pulse output port DPLS10 for outputting a pulse, and one A/D port CRINT. The port PWM shown in FIG. 6 is not provided in the microcomputer IC301.
The pulse output from the port DPLSVAR of the microcomputer IC301 is generated by frequency division using a digital circuit counter. A pulse having one of 256 frequencies is output from the port DPLSVAR, for example, as shown in FIG. 10. The pulse has a waveform with the ON duty varying from 25% to approximately 1%. With respect to the variations in the pulse output from the port DPLSVAR of the microcomputer IC301, the output voltage of the positive transfer voltage generator circuit 102 changes as shown in FIG. 11.
The positive transfer voltage generator circuit 102 thus has fewer components when constituted of the circuit disclosed in Japanese Patent Application Laid-Open No. H08-140351 as compared with the transfer voltage generator circuit 202 shown in FIG. 6. The transfer voltage generator circuit 301 can therefore be configured at low cost.
However, when the circuit disclosed in Japanese Patent Application Laid-Open No. H08-140351 is used as the positive transfer voltage generator circuit 102, the pulse frequency for driving the inverter transformers becomes low in the transfer voltage generator circuit 101 in a case where the required transfer voltage becomes low. An output ripple in the transfer voltage therefore becomes large. Furthermore, the digital circuit counter must be added in order to generate low frequency pulses for driving the inverter transformers.